1. Field of the Invention
This invention relates to devices for sensing selected conditions in confined areas and/or in combination with moving parts and, more particularly, to devices for sensing the temperature of a piston in an internal combustion engine.
2. Description of the Prior Art
This invention will be described as embodied in apparatus for monitoring piston temperatures in an internal combustion engine. It will be appreciated that a number of other applications are possible, for example to measure strain or vibration in a member or measure the temperature condition of a turbine blade.
There are a number of available technologies for sensing piston temperatures, but a major problem is to get the sensed condition out of the cylinder as a reliable, usable signal. For example, it has been proposed to bring the signal out of the engine using slip rings, radio telemetry, and mechanical linkages. These have a number of inherent problems. Slip rings are susceptible to electrical noise which affects reliability. Radio telemetry units are generally heavy and adversely affected by high "g" forces and elevated temperatures. Mechanical linkages are difficult to install, requiring extensive modifications to the engine, and are limited as to the speeds to which they can be exposed, i.e. engine RPM.
Another previous method proposed for measuring the temperature of a piston in an internal combustion engine is to incorporate a plug in the piston wherever a temperature reading is required. These plugs are made of a specially treated alloy capable of withstanding elevated temperatures for a specific period of time. But certain characteristics of the plug, e.g. hardness, are affected by the elevated temperatures, and changes in those characteristics are used to arrive at temperature measurement.
An example of this type of measurement system is disclosed in U.S. Pat. No. 4,787,282 issued Nov. 29, 1988 to Shine. In this patent an Aluminum-Copper alloy plug is incorporated into the piston. This plug can be a band integrally cast with the piston, a setscrew, or a wedge that is press-fit into the piston. This plug is subjected to a qualifying run of at least 100 hours and the hardness of the plug is then measured. The steady-state temperature of the piston is then calculated by reference to a calibration curve graphing hardness verses temperature for the given alloy mixture. A problem with this type of piston measuring device is that it will not respond to fast temperature transients on the surface of the piston. Further, these devices must be removed from the piston, sent to a laboratory to be tested for hardness, and then the temperature of the piston calculated from the calibration curve. This is a very time consuming process and is unacceptable where the temperature of the piston is needed immediately.
Devices of the type to which this invention relates will find important application as a research tool in selecting piston/cylinder materials and formulating fuel and lubrication products.